1. Field of the Invention
The present invention relates to a developer concentration detection apparatus, and more particularly, to a developer concentration detection apparatus which can measure a concentration of a developer by measuring an optical transmissivity of a developer in an electrophotographic printer.
2. Description of the Related Art
Generally, a photosensitive medium such as a photosensitive belt or photosensitive drum is used in an electrophotographic printer. As is known, the surface of the photosensitive medium can be charged and the level of the electric potential can be selectively changed by a scanned beam, thereby forming an electrostatic latent image. The printers are divided into a dry type and a wet type according to the state of a toner attached to the electrostatic latent image. In a wet type printer, a developer obtained by mixing a liquid toner and a carrier is used in printing.
FIG. 1 shows a schematic view of a general wet type color printer. Referring to FIG. 1, the printer includes a photodetection belt 14 which is supported by a driving roller 11, a direction control roller or tension roller 12, and a counter roller 13, and rotates along an endless track, at least one of the developing units 17a, 17b, 17c and 17d, and an image transfer roller 25. The photodetection belt 14 can be maintained in a state where the surface thereof has been charged by a corona apparatus 16. The charged state of the photodetection belt 14 can be changed in electric potential as laser beams are scanned from laser scanning units 18a, 18b, 18c and 18d which are closely installed to the respective developing units 17a, 17b, 17c and 17d. The beams scanned from the laser scanning units 18a, 18b, 18c and 18d form latent images on the surface of the photodetection belt.
The developing units 17a, 17b, 17c and 17d can develop colors of black, yellow, cyan and magenta, according to the paint color. Also, the laser scanning units 18a, 18b, 18c and 18d can form latent images with respect to corresponding colors. Developers are filled in the developing units 17a, 17b, 17c and 17d. The developer can be attached to a latent image formed on the surface of the photodetection belt 14 via a developing roller (not shown) provided in each developing unit. A backup roller (not shown) is mounted in correspondence to each developing roller (not shown). A cleaning roller 23 has the function of completely removing the carrier from the toner image formed on the photodetection belt 14. A heating roller 24 evaporates the carrier in order to assist the function of the cleaning roller 23. The toner image attached to the photodetection belt 14 is transferred to the image transfer roller 25, so that the image can be transferred from the image transfer roller 25 to paper 27. The printing paper 27 passes between the image transfer roller 25 and a fixing roller 26. An erasure 15 performs the function of completely removing the charging state remaining on the photodetection belt 14, to thereby form a new electrostatic latent image.
The developer filled in each developing unit 17a, 17b, 17c or 17d is a mixture of a toner and a carrier. The toner is stored in toner containers 19a, 19b, 19c and 19d, and supplied to a mixture containers 21a, 21b, 21c and 21d via pumps 20a, 20b, 20c and 20d, respectively. The carrier is stored in a carrier container 28, and supplied to mixture containers 21a, 21b, 21c and 21d, respectively. The mixture containers 21a, 21b, 21c and 21d mix the toner and carrier at a proper concentration and supply the mixed result to each developing unit 17a, 17b, 17c or 17d via the pumps 22a, 22b, 22c and 22d. The carrier container 28 also stores the carrier returning from the cleaning apparatus including the cleaning roller 23 and the heating roller 24.
In the above printer, the concentration of the developer supplied to each developing unit 17a, 17b, 17c or 17d should be maintained constant. That is, a ratio of the mixture of the toner and carrier should be maintained properly. This is performed by controlling the amount of liquid supplied from the toner containers 19a, 19b, 19c and 19d and the carrier containers 28 to the mixture containers 21a, 21b, 21c and 21d, respectively. Also, in order to control a ratio of the mixture of the toner and carrier, an apparatus for detecting a concentration of the developer should be provided. According to the prior art, developer concentration detection apparatuses are disclosed in U.S. Pat. Nos. 4,310,238 (Ricoh) and 5,570,193 (Indigo). These apparatuses are not widely used since the measuring method is changed according to the characteristic of the developer.